In recent years, as a technique for increasing an aperture ratio greatly, it has been suggested that a back-illuminated type structure on which light is incident from a second surface opposite from interconnect layers can be applied to CMOS image sensors. In the back-illuminated type structure, the interconnect layers, transistors for reading, and the like are formed on a first surface of a semiconductor substrate. A photodiode array including photodiodes that photoelectrically convert light into signals is formed on the second opposite from the first surface. The second surface is the illuminated surface. Color filters and the like that divide incident light wavelengths into wavelength ranges such as R (red), G (green), and B (blue), for example, are formed on the illuminated surface, and a microlens for gathering light is further formed on the color filters and the like.
The photodiode array formed on the illuminated surface includes photodiodes formed with silicon single crystals, but a large number of defect levels exist at the interface between silicon forming the photodiodes and a silicon oxide film formed over the photodiodes. Due to the defect levels, electrons that are not derived from signals generated through photoelectric conversion are generated by thermal excitation or the like, and dark noise components called “dark current” increase.
To reduce the dark current, a transparent electrode is formed on the illuminated surface, and biasing is performed by applying a negative voltage. In this manner, holes are stored on the interface, and electron excitation at the interface can be restrained. In such a case, however, a negative power supply is required, and the device becomes complicated.